Strangely I find the easiest way to get a foothold is just kill the Dutch. Then Ceylon turns Rebel and you get Amsterdam to boot. That, and Dutch Guyana turns rebel aswell which is a pretty wealthy province.
Personally, i've found the AI doesn't really accept provinces for anything else other than other provinces. For example, the Venitians in my most recent campaigns were blockading my damn ports after I took Venice and refused a ceasefire. So I sailed to Greece and killed them off. Now, I don't really want a border with the Ottomans, so I decided to try and sell them the province for cash. 5000? Nope. 3000? Nope. 2000? Nope. Eventually they get bored and accept to pay me 1000 for it. Considering it makes 1k a turn I got pretty ripped off, but the AI seemed to refuse to consider anything else. Albeit, that was selling, not buying.
Though a funny thing in that campaign was what France were doing. I controlled Savoy, and they kept exchanging me provinces for it. First, the Windward Islands, with about 4 technologies and 500 gold. I accept. Next turn, I immediatly conquer back Savoy, which has no standing army. France accept a ceasfire, and what's more, they didn't call Spain into the war. France gives me a similar offer with another Caribbean Island (the one the Dutch hold, which they swapped for Guyana), along with 2 techs and some small amount of gold. I accept, and reconquer, and force a ceasefire. Then, they give me Newfoundland, and 2 more techs and some more gold for it. I conquer it back again, and they immediatly accept a ceasefire, and no Spanish inolvement again! So effectivly, i've scammed France for 8 technologies and 3 colonies and about 1.2k gold or so. Lovely

[Now however, with no more islands to trade, they offer me a ... trade agreement for Savoy... hell no]. And all it's cost me is the French are stuck in Hostile, which doesn't bother me at all
